Anchor Scope - Newbie Question

[jdc]

So are you saying that the Navy have anchored warships without anchors successfully then?

Err, no. What gave you that impression? What I'm saying is that on big boats the effect of the chain in making the pull at the anchor horizontal is real, whereas in small boats and shallow water (and the two often go together) the effect is much less. In either case it's the anchor which holds the horizontal load!

 

[davidej]

On page 5 now

Nothing gets the peeps going better than an anchor thread.

 

[KellysEye]

>Come on - a very moderate wind (F3?) can lift the chain off the ground so the anchor almost always takes some strain. I wonder whether this "chain holds the boat" myth comes from

All I can say is we anchord hundreds of times in the clear water of the Caribbean where you could see the anchor by swimming over it. Using the amounts of chain I said I never saw the chain pulling the anchor. If it does there isn't enough chain our or it's a rope chain mix which I've seen swimming over other boats anchors.

 

[lustyd]

>Come on - a very moderate wind (F3?) can lift the chain off the ground so the anchor almost always takes some strain. I wonder whether this "chain holds the boat" myth comes from

All I can say is we anchord hundreds of times in the clear water of the Caribbean where you could see the anchor by swimming over it. Using the amounts of chain I said I never saw the chain pulling the anchor. If it does there isn't enough chain our or it's a rope chain mix which I've seen swimming over other boats anchors.

Next time try swimming down and removing the anchor and see if you really are right

 

[noelex]

Using the amounts of chain I said I never saw the chain pulling the anchor. If it does there isn't enough chain our or it's a rope chain mix which I've seen swimming over other boats anchors.

Sorry but how do you know if the chain is "pulling on the anchor" or not ?

 

[zzyyxx]

Lots has been said in favour of putting out more scope than the RYA recommended (4x chain - 6x mixed), but is it not anti-social where other people might want to use the anchorage?

If people stick within a standard range then there'll be less chance of a surprise when the chains collide at the turn of the tide won't there?

 

[vyv_cox]

Lots has been said in favour of putting out more scope than the RYA recommended (4x chain - 6x mixed), but is it not anti-social where other people might want to use the anchorage?

I remember watching a German-crewed boat, about 45 ft, in Soller, Mallorca. They started going astern as close to the upwind shore as possible, ridiculous speed - more than 5 knots. Dropped the anchor and powered through the anchorage, which was already quite crowded. We estimated that they ran 100 metres of chain out, water depth 5 metres or a bit more. Within a minute of stopping and turning the engine off they were in the tender and heading ashore. Less than ten minutes later their boat had swung and collided with an adjacent one. Later the wind changed direction and they ran aground.

 

[jimbaerselman]

Lots has been said in favour of putting out more scope than the RYA recommended (4x chain - 6x mixed)

Rash to make a simple recommendation like that. I would guess that such a statement was qualified by "average boat in average conditions in UK waters"

Boat anchors pull out of the ground if the pull angle is too steep. "Too Steep" varies depending on the anchor design being used.

The upward pull angle at the anchor depends on the force being applied, the depth, the scope deployed, chain weight, and bottom slope or whether it caught an obstruction. Ignoring bottom slope and obstructions, you'll only find the minimum scope to provide best holding for your particular combination by testing against full astern (sailboat). Spend an hour one day trying ever decreasing scopes on a level bottom to see what that is. I've found some combinations call for x7, others X4.5, mainly dependent on anchor type.

Whether you choose to use that much or not is local judgement, depending on the probability of a wind increase. But it is useful to know that you don't need more than "whatever "

 

[KellysEye]

>Sorry but how do you know if the chain is "pulling on the anchor" or not ?

If you look down to the bottom of the catenary and there is a long bit of chain lying flat on the bottom then there is no load on the anchor. If it is curved all the way to the anchor then there is load on it.

 

[noelex]

>Sorry but how do you know if the chain is "pulling on the anchor" or not ?

If you look down to the bottom of the catenary and there is a long bit of chain lying flat on the bottom then there is no load on the anchor. If it is curved all the way to the anchor then there is load on it.

That is not correct.
There can still be a significant force on the anchor when the chain is horizontal.

When the rode angle is above horizontal the holding power of the anchor is reduced, but this should not to be confused with concluding that there is no force on the anchor when the chain is horizontal.

 

[KellysEye]

>When the rode angle is above horizontal the holding power of the anchor is reduced, but this should not to be confused with concluding that there is no force on the anchor when the chain is horizontal.

If you want to check if that is true dive down to the chain and pick it up to feel if there is any load on it, there isn't, I've done that.

I meant mention that somebody said that putting no load on an anchor can be done by the Navy because of the large chain they use, they are large ships. My bet is that the size/weight ratio of chain for yachts/ships is the much the same.

 

[nigelmercier]

Ask 20 sailors how much scope to use and you will get 20 different answers ...

 

[Neeves]

>


If you want to check if that is true dive down to the chain and pick it up to feel if there is any load on it, there isn't, I've done that.


Hi KE,

I'm interested, how do you measure, identify, there is no load on the anchor?

If I put a load cell into the system and I increase the load such that the load lifts the chain off the seabed upto the point where the last link is still touching the bottom, (seabed) then the chain at the anchor is horizontal, though only for one link. I have a load at one end (30m x 8mm chain 5:1 scope) of 80kg - where precisely does that load go (if it is not imposed on the anchor?). I reduce the load to 75kg, and have a few more links on the seabed - where exactly does that load go - it is impossible to have 75kg (or 80kg) at one end of a chain and not have a load at the other end.


I confess I am fascinated.

Jonathan

 

[vyv_cox]

I meant mention that somebody said that putting no load on an anchor can be done by the Navy because of the large chain they use, they are large ships. My bet is that the size/weight ratio of chain for yachts/ships is the much the same.

I was talking to a RN ex-deck officer last year. He served on aircraft carriers and told me they always dragged. An officer (sometimes him) was stationed to observe when they had dragged to the prescribed limit, when they would haul anchor, motor back upwind and re-anchor. Doesn't seem to have been only in strong winds, although clearly not in a flat calm, unless there was a bit of tide.

 

[jimbaerselman]

- where exactly does that load go - it is impossible to have 75kg (or 80kg) at one end of a chain and not have a load at the other end.

That's the trouble with tension - it will go two ways!

Now, I wonder what the coefficient of friction is for a link lying on the sea bed . . . multiplied by the number of links plus the (underwater) weight of those links . . . I now have a picture in my mind of Vyv Cox marching across the hard with 60m of chain trailing behind . . .

 

[oldbilbo]

I was talking to a RN ex-deck officer last year. He.... told me they always dragged. An officer (sometimes him) was stationed to observe when they had dragged.

I've sailed with several ex-RN Seaman Speciality officers, and every last one of 'em veered insufficient chain/rode to keep their benighted boats in one place. Every last one of 'em insisted on ranging the chain out on deck preparatory to 'coming to' an anchor berth - sometimes a couple of miles or more to seaward....

Never mind a jackstay rolling underfoot - try getting forward coming into New Grimsby Harbour, IoS, 'at the rush' with 150' of 1/2 chain sliding about on the foredeck! Hooneedzum...?

 

[KellysEye]

>I reduce the load to 75kg, and have a few more links on the seabed - where exactly does that load go - it is impossible to have 75kg (or 80kg) at one end of a chain and not have a load at the other end.

Sorry I don't think I was quite clear. If the chain is on the bottom by definition there is a vertical load on it (if you lift it by hand) but not angled. However if there isn't enough chain out the boat snatch load when swinging would pull the chain off the bottom and put load on the anchor. What we do as I mentioned is make sure the weight of the catenary will hold the boat, thus no chain angled snatch load on the anchor. We also use a riding sail at anchor as it cuts down the swinging by about 30% and thus reduces the load on the catenary by 30%. I am suprised how few boats use one, I don't think I've ever seen one in the UK. Hope that all makes sense.

>[aircraft carriers and told me they always dragged.

I wonder if they can't carry enough chain because of the weight. On the other hand with a big crew and people on watch night and day I don't suppose it matters if they drag provided they have room behind them.

 

[Roberto]

If I put a load cell into the system and I increase the load such that the load lifts the chain off the seabed upto the point where the last link is still touching the bottom, (seabed) then the chain at the anchor is horizontal, though only for one link. I have a load at one end (30m x 8mm chain 5:1 scope) of 80kg - where precisely does that load go (if it is not imposed on the anchor?). I reduce the load to 75kg, and have a few more links on the seabed - where exactly does that load go - it is impossible to have 75kg (or 80kg) at one end of a chain and not have a load at the other end.

there are three forces acting on a static catenary, not two: the tension/pull at the boat extremity, the horizontal pull at the anchor AND the weight of the chain.
In equilibrium, the sum of the *three* forces must be zero.
In practical terms, the sentence "if I pull 100 on the chain then I pull 100 on the anchor" is almost never true.

I think some of the confusion arises from people mixing up between the load of external forces on the boat, which is mainly horizontal, and the tension/pull on the anchor chain, which has an angle: for a given force exerted by the wind the load on the chain will be higher as it is angled (divided by a sin/cos which is <1).
If you pull 80kg on the chain at the angle the chain forms from the davit, one component of that is balanced by the hull floatation, another component by the horizontal load on the boat. With a tangent catenary (last link on the seabed) the pull on the anchor is a lot less than the 80kg you are pulling, the (vectorial) difference being the weight of the chain.
In other words, the 80kg one is pulling from the boat will be roughly 80kg at the anchor with an almost bar-taut chain (in practical terms); with any sensible slack of the chain the load on the anchor will be a lot less than the 80kg.

Sometimes the wind drag of the boat is equated with the pull on the chain, which is nonsense of course.
At equilibrium with a tangent chain, the wind drag is equal to the pull on the anchor (they are both horizontal), *but* the load on the chain will be a lot higher: 100daN wind drag with a 30° angle of the chain gives 200daN on the chain. In terms of vectors, the 200 at 30° are the same as an horizontal vector of 100 (counteracting the wind drag) and a vertical one of about 170 (counteracting the weight of the chain, which is beared by the boat floatation).
So if your cell load on the anchor chain measures a given load, it is very, very likely that the anchor pull will be a lot less than that.


All in purely static terms of course.

 

[Boo2]

I was talking to a RN ex-deck officer last year. He served on aircraft carriers and told me they always dragged. An officer (sometimes him) was stationed to observe when they had dragged to the prescribed limit, when they would haul anchor, motor back upwind and re-anchor. Doesn't seem to have been only in strong winds, although clearly not in a flat calm, unless there was a bit of tide.

Interesting. We moored at Falmouth in 2011 and saw lots of big ships at anchor but they'd driven right up to their anchors so the chain was practically straight up and down, what's the thinking behind that doe sanyone know ? Or have I misunderstood what was going on ?

Boo2

 

[zzyyxx]

I've sailed with several ex-RN Seaman Speciality officers, and every last one of 'em veered insufficient chain/rode to keep their benighted boats in one place. Every last one of 'em insisted on ranging the chain out on deck preparatory to 'coming to' an anchor berth - sometimes a couple of miles or more to seaward....

Never mind a jackstay rolling underfoot - try getting forward coming into New Grimsby Harbour, IoS, 'at the rush' with 150' of 1/2 chain sliding about on the foredeck! Hooneedzum...?

I did my first sailing course - a two day flot. thing - with an ex-RN Commander, we grounded within an hour the first day (possibly deliberate) and he similarly laid out chain.
Everyone I've sailed with since looks at me like I'm mad if I suggest such a thing...

It did work, though t'was too short a course for it to get properly tested.